The present invention relates to a magnetic recording medium comprising a magnetic layer having ferromagnetic powder and a binder dispersed in it, said magnetic layer being placed on a non-magnetic support member, and in particular, to a magnetic recording medium having excellent electromagnetic transfer characteristics and durability.
Magnetic recording medium is now widely used as recording tape, video tape, floppy disk, etc. Magnetic recording medium comprises a magnetic layer where ferromagnetic powder is dispersed in a binder, and said magnetic layer is placed on a non-magnetic support member.
Magnetic recording medium must be at high level in the properties such as electromagnetic transfer characteristics, running durability, and running performance. Specifically, an audio tape for reproducing musical sound must have high ability to reproduce original sound, and a video tape must have high electromagnetic transfer characteristics such as high ability to reproduce original picture.
In addition to high electromagnetic transfer characteristics as described above, magnetic recording medium must have good running durability. To obtain good running durability, abrasive material and lubricant are generally added to the magnetic layer.
In the equipment or device using the magnetic recording medium, there have been problems in that, when medium slides on and contacts a magnetic head, low molecular weight components in the binder of the magnetic recording medium migrate toward surface of the magnetic layer and are attached on the magnetic head. As a result, the magnetic head is often contaminated.
The contamination of the magnetic head causes deterioration of the electromagnetic transfer characteristics. In particular, in an equipment or a device for high density recording, number of revolutions of the magnetic head is high, being 9,600 rpm in case of a digital video tape recorder, 1,800 rpm in case of an analog video tape recorder for home use, and 5,000 rpm for a tape recorder for business use. Thus, sliding speed of the magnetic recording medium on the magnetic head is increased, and there is also a trend that the magnetic head is designed more and more in compact size such as thin film head. In this respect, there are strong demands to reduce and eliminate the contamination of the magnetic head caused by the components from the magnetic recording medium.
To solve the above problems, it is now practiced to increase hardness of the magnetic layer using a hard binder. Further, it is attempted to decrease the content of the low molecular weight component in the binder to prevent resin components in the binder from attaching on the magnetic head and from causing contamination of the head.
For example, JP-A-62229523 discloses a method for removing low molecular weight components having molecular weight of not more than 2,000, whereby magnetic coating material is coated and dried on a magnetic recording medium and the magnetic recording medium is then exposed in a supercritical fluid with temperature and pressure higher than critical values and low molecular weight components are extracted and removed using the supercritical fluid. However, this requires a pressure vessel, which is resistant to high temperature and high pressure, and this is disadvantageous for the use in industrial application. Also, there is a problem in that it is difficult to evenly remove the low molecular weight components from the binder in the magnetic recording medium, which is in wound state.
JP-A-63263629 (U.S. Pat. No. 5,153,063) discloses a method to reduce the low molecular weight components to less than 3 weight % by the methods such as: (1) conditioning of a polymerizing catalyst; (2) increase of molecular weight of the resin itself; and (3) fractional precipitation using poor solvents such as alcohols, aromatic compounds, aliphatic hydrocarbons, etc. after synthesis. More concretely, a method of fractional precipitation using hexane or a mixture of hexane/toluene (7:3) is described. However, when hexane is used, solubility of dilute phase is low, and the low molecular weight components cannot be removed completely. In case the hexane/toluene mixture solvent is used, solubility can be adjusted by the content of toluene, while yield is decreased because high molecular components are also dissolved.
Further, JP-A-0652539 discloses a method to reduce the context of the low molecular weight components having molecular weight of 400 to 1,300 to 0.5 to 2.5 weight OI, using the methods such as (1) re-precipitation in organic solvent such as toluene after synthesis of polyester polyurethane; (2) use of polyurethane purified by re-precipitation in organic solvent such as toluene; and (3) synthesis of raw material polyurethane having such structure as to prevent generation of low molecular weight components (such as long-chain diol, and cyclic diol). In the embodiments, the change to synthetic method not using diol as chain extender is described.